1. Field of the Invention
This invention relates to a torque converter with a lock-up mechanism in an automatic transmission for a vehicle. More particularly, it relates to a torque converter with a lock-up mechanism which can achieve an improvement in transmission torque capacity by a simple construction and can prevent vibration (judder) created by stick-slip.
2. Related Background Art
Generally, a torque converter can realize smooth running because it transmits power through a fluid, while it suffers from the disadvantage that fuel consumption is aggravated because of the energy loss due to the slip of the fluid. In order to overcome this, the latest torque converters are provided with a lock-up mechanism.
The lock-up mechanism is a mechanism which comprises a direct-coupled clutch (lock-up clutch) and in which when the speed of vehicle reaches a predetermined or higher speed, the flow of the fluid in the torque converter automatically changes to urge the friction surface of a frictional material attached to the piston of the direct-coupled clutch against the front cover of the torque converter to thereby directly couple an engine and a drive wheel together. Thereby, the influence of the slip of the fluid is eliminated and therefore, an improvement in fuel consumption can be achieved.
When the piston (lock-up piston) of the direct-coupled clutch is fastened to the front cover of the torque converter, a pressure force by hydraulic pressure is acting on the sliding portion, between the friction surface of the frictional material attached to the piston and the front cover. To keep the fastened state of the piston and the front cover in the sliding portion good, it is necessary to increase the pressure force by the hydraulic pressure. Accordingly, in the fastened state, high pressure is being applied to the frictional material.
When the pressure force becomes higher, excess reaction due to the distribution of the hydraulic pressure becomes greater thereby and the possibility of imparting an adverse effect to the fastened state of the friction surface of the piston and the front cover becomes high and therefore, as a countermeasure for preventing this, it has been practiced to form a groove in the surface of the frictional material as described in Japanese Laid-Open Utility Model Application No. 1-128057 or to form a groove in the surface of the piston as described in Japanese Laid-Open Patent Application No. 6-346951.
In recent years, it has often been the case that slip control is adopted for the lock-up clutch with a view to improve fuel consumption, but judder vibration occurs during slip. Therefore, it is necessary to prevent judder vibration and as a measure for preventing it, it is practiced to subject the friction surface of the lock-up clutch to a predetermined amount of cutting work and in addition, use low ps oil to make .mu.-V characteristic into a positive gradient (note: a negative gradient would cause judder vibration).
Such an example is described, for instance, in Japanese Laid-Open Patent Application No. 5-99297. FIGS. 23 and 24 of the accompanying drawings are views for illustrating this example of the prior art, and an annular frictional material 140 is attached to the front cover side of the piston 100 of a lock-up clutch, and as shown in FIG. 24, the surface of the frictional material 140 from the radially outer peripheral edge portion A thereof to the intermediate point B thereof is subjected to cutting work between the outer peripheral edge portion A and the inner peripheral edge portion C. In FIG. 23, it is an annular area 130. By doing so, any stick-slip occurring during the liberation and fastening of the lock-up clutch can be prevented.
In the above-described construction, however, when the engagement hydraulic pressure changes from low pressure to high pressure, the area of contact and Rm (average effective radius) become smaller as the manner of bearing of the friction surface shifts from the bearing against the outer diameter to the bearing against the inner diameter (see FIG. 25 of the accompanying drawings). That is, by the hydraulic pressure coming round from the outer peripheral portion of the frictional material into between the friction surfaces as indicated by arrow in FIGS. 25 and 26 of the accompanying drawings, a distribution of hydraulic pressure is created radially and circumferentially of the frictional material 140 to thereby reduce the piston thrust which brings the piston 100 into pressure contact with the front cover 200. Accordingly, due to these factors, there has arisen the problem that during high hydraulic pressure, transmission torque capacity is reduced (see FIG. 26). FIG. 25 shows-the manner in which during low hydraulic pressure, the frictional material 140 bears against the front cover 200 on the outer diameter side, and FIG. 26 shows the manner in which during high hydraulic pressure, the frictional material 140 bears against the front cover 200 on the inner diameter side.
The countermeasure by the above-described construction is effective for the prevention of judder, but may result in a reduction in the transmission torque capacity during high hydraulic pressure. As measures for improving the transmission torque capacity, it would occur the diameter of the clutch larger, to construct the clutch of multiple plates and to increase the hydraulic pressure, but this would conversely make the structure of the lock-up clutch complicated and bulky and still, would result in an increase in fuel consumption.
For such a reason, it has heretofore been very difficult to prevent any reduction in the transmission torque capacity during high hydraulic pressure by a combination of a lock-up clutch having its friction surface subjected to a set amount of cutting work and low .mu.s oil.
When a pressure force becomes high, the excess reaction due to the distribution of hydraulic pressure also becomes great, and this may adversely affect the joined state of the friction surface of the piston and the front cover and also may impart damage such as deformation to the torque converter itself.
Particularly, it is important to decrease the distribution of hydraulic pressure and increase the piston thrust in order to prevent any reduction in transmission torque capacity during the operation of the lock-up clutch at high hydraulic pressure.